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At least 235 records · Page 13Linked to original sources
PubMed2005

Thermotropic phase transition in soluble nanoscale lipid bilayers.

The role of lipid domain size and protein-lipid interfaces in the thermotropic phase transition of dipalmitoylphosphatidylcholine (DPPC) and dimyristoylphosphatidylcholine (DMPC) bilayers in Nanodiscs was studied using small-angle X-ray scattering (SAXS), differential scanning calorimetry (DSC), and generalized polarization (GP) of the lipophilic probe Laurdan. Nanodiscs are water-soluble, monodisperse, self-assembled lipid bilayers encompassed by a helical membrane scaffold protein (MSP). MSPs of different lengths were used to define the diameter of the Nanodisc lipid bilayer from 76 to 108 A and the number of DPPC molecules from 164 to 335 per discoidal structure. In Nanodiscs of all sizes, the phase transitions were broader and shifted to higher temperatures relative to those observed in vesicle preparations. The size dependences of the transition enthalpies and structural parameters of Nanodiscs reveal the presence of a boundary lipid layer in contact with the scaffold protein encircling the perimeter of the disc. The thickness of this annular layer was estimated to be approximately 15 A, or two lipid molecules. SAXS was used to measure the lateral thermal expansion of Nanodiscs, and a steep decrease of bilayer thickness during the main lipid phase transition was observed. These results provide the basis for the quantitative understanding of cooperative phase transitions in membrane bilayers in confined geometries at the nanoscale.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed2005

In-line monitoring of partial and overall solid concentration during solvent-mediated phase transition using Raman spectroscopy.

A calibration strategy for the continuous monitoring of solvent-mediated phase transition was developed using in situ Raman spectroscopy. Citric acid which exhibits enantiotropy during its anhydrous/monohydrate phase transition was selected as a model organic product. Using 25 samples in suspension, specific calibration of the spectral data was obtained for estimating in-line both the overall solid concentration in suspension and the composition of the solid phase. In addition to such key-measurements, reliable estimates of supersaturation were computed in-line from the mass balance of the solute. In order to validate the technique, anhydrous to monohydrate phase transition experiments were performed in suspensions at 15 degrees C and the kinetic process involved was monitored. Despite the use of various solid concentrations and reactor configurations, both the reproducibility and the reliability of the in situ Raman measurements are shown to be satisfactory.

Calibration↗
PubMed1996

Depth dependence of the perturbing effect of placing a bulky group (oxazolidine ring spin labels) in the membrane on the membrane phase transition.

Electron paramagnetic resonance (EPR) and differential scanning calorimetry (DSC) have been used to study the effect on the phase transition of dimyristoylphosphatidylcholine membranes of incorporating various stearic acid spin labels (SASL's) that contain the bulky oxazolidine ring at various positions along the stearyl chain. SASL's lowered the phase transition temperature and decreased the size of the cooperative unit, with the effects stronger in the order of 9- > 12- > 5- > 16-SASL > stearic acid (no label). Incorporation of stearic acid without the spin label slightly increases the phase transition temperature. Incorporation of 9-SASL (3 mol% of lipid) decreased the transition temperature by 1.8 degrees C and the cooperative unit to 1/5 of that without the spin label, while the effect of 16-SASL was slight. The effect on transition enthalpy was small. It is concluded that the perturbing effect of placing a bulky group on the alkyl chain on phase transition is through inducing packing defects in the gel-phase.

Calorimetry, Differential Scanning↗
PubMed2002

Phase transition of a two-dimensional binary spreading model.

We investigated the phase transition behavior of a binary spreading process in two dimensions for different particle diffusion strengths (D). We found that N>2 cluster mean-field approximations must be considered to get consistent singular behavior. The N=3,4 approximations result in a continuous phase transition belonging to a single universality class along the D subset (0,1) phase transition line. Large scale simulations of the particle density confirmed mean-field scaling behavior with logarithmic corrections. This is interpreted as numerical evidence supporting the bosonic field theoretical prediction that the upper critical dimension in this model is d(c)=2. The pair density scales in a similar way but with an additional logarithmic factor to the order parameter. At the D=0 end point of the transition line we found directed percolation criticality.

Journal Article↗
PubMed1986

Dimyristoylphosphatidic acid/cholesterol bilayers. Thermodynamic properties and kinetics of the phase transition as studied by the pressure jump relaxation technique.

Lipid bilayers and monolayers composed of dimyristoylphosphatidic acid (DMPA) and cholesterol were characterized by differential scanning calorimetry and film balance measurements. Increasing cholesterol content decreases the bilayer phase transition temperature and enthalpy in a manner similar to that observed before for other lipid/cholesterol systems. In monomolecular films at the air-water interface cholesterol exhibits the well known condensing effect in the liquid-expanded phase, while the liquid-condensed phase is less affected. As with the bilayer phase transition, the transition temperature and change in area at the liquid-condensed to liquid-expanded phase transition, as measured from isobars at 25 dynes/cm, decreases with increasing cholesterol content. The kinetics of the phase transition of DMPA/cholesterol bilayers were measured using the pressure jump relaxation technique with optical detection. Three relaxation times were observed. The relaxation times and amplitudes pass through maximum values at the transition midpoint. With increasing cholesterol content the maximum values of the relaxation times decrease but not in a linear fashion. The time constants display an intermediate maximum at ca. 10% to 12 mol% cholesterol. This observation is discussed in terms of a possible change in the nature of the phase transition from first-order with phase separation to a continuous second-order transition. The dependence of the relaxation amplitudes on cholesterol content gave evidence for nucleation being the rate limiting step for the transition in this particular system.

Calorimetry, Differential Scanning↗
PubMed1982

Delivery of antitumor drug to bladder cancer by use of phase transition liposomes and hyperthermia.

It has been proposed that liposomes (phospholipid bilayer vesicles) could be used to entrap enzymes or drugs and then be administered intravenously to patients as a natural molecule to be metabolized. After liposome degradation, the entrapped molecules would be released at higher concentrations in target tissues. In 1979, it was shown that liposomes containing a chemotherapeutic agent could preferentially release the agent at the transition temperature of the lipids. In an effort to deliver high dose methotrexate (MTX) to animal transitional cell carcinomas (TCC), we used liposomes with phase transition temperature a few degrees above 37 C. Experimentally induced bladder TCC tumors (induced with N-(4-(5-nitro-2-furyl)-2 thiazolyl) formamide) were transplanted into hind legs of C3H/Bi mice. These animals received tritiated MTX encapsulated in phase transition liposomes. Tumors were heated to 42 +/- 0.5 C (by ultrasound) before receiving the liposomes containing tritiated molecules of MTX. After an appropriate time sequence the tumors were removed and MTX uptake of each tumor was noted and compared to the controls having unheated tumors. Heated TCC tumors accumulated 11.9-fold more MTX than nonheated tumors receiving the same dose. Animals receiving free MTX did not exhibit a temperature dependent difference. This indicates that the phase transition temperature of specifically engineered liposome may be used to increase delivery of high dose methotrexate to transitional cell carcinomas.

Animals↗
PubMed2006

Phase transition behavior of unimolecular micelles with thermoresponsive poly(N-isopropylacrylamide) coronas.

This paper describes the double phase transition behavior of a thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) brush at the surface of a hydrophobic core. Reversible addition-fragmentation transfer (RAFT) polymerization of N-isopropylacrylamide (NIPAM) was conducted by using a hyperbranched polyester (Boltorn H40) based macroRAFT agent. The resultant multiarm star block copolymer (H40-PNIPAM) exists as unimolecular micelles with hydrophobic H40 as the core, densely grafted PNIPAM brush as the shell. A combination of laser light scattering (LLS) and microdifferential scanning calorimetry (micro-DSC) studies of H40-PNIPAM in aqueous solution reveals double phase transitions of the PNIPAM corona, which is in contrast to the fact that free PNIPAM homopolymer in aqueous solution exhibits a lower critical solution temperature (LCST) at approximately 32 degrees C. The first phase transition takes place in the broad temperature range 20-30 degrees C, which can be tentatively ascribed to the n-cluster-induced collapse of the inner region of the PNIPAM brush close to the H40 core; the second phase transition occurs above 30 degrees C, which can be ascribed to the outer region of PNIPAM brush. Employing the RAFT chain extension technique, the inner and outer part of PNIPAM brush were then selectively labeled with pyrene derivatives, respectively; temperature-dependent excimer fluorescence measurements further support the conclusion that the inner part of PNIPAM brush collapses first at lower temperatures, followed by the collapse of the outer part at higher temperatures.

Journal Article↗
PubMed2003

Mechanism of the first-order phase transition of an acylurea derivative: observation of intermediate stages of transformation with a detailed temperature-resolved single-crystal diffraction method.

The process of the first-order solid-to-solid phase transition of 1-ethyl-3-(4-methylpentanoyl)urea (1) was observed by means of a detailed temperature-resolved single-crystal diffraction method, which resembles watching a series of stop-motion photographs. The transition consists of two elementary processes, one supramolecular and the other molecular. Crystal structures from before and after the phase transition are isostructural. The straight-ribbon-like one-dimensional hydrogen-bonding structure is formed and stacked to form a molecular layer. The geometry of the layer is retained during the phase transition. The relative position of the layer with its neighbours, on the other hand, changes gradually with increasing temperature. The change is accelerated at the temperature representing the start of the endotherm seen in the DSC curves of (1). The structural variation yields void space between the neighbouring layers. When the void space grows enough that the crystal is unstable, the 3-methylbutyl group on the last of the molecules turns into a disordered structure with drastic conformational changes to fill up the void space. The phase transition process is well supported with simple force-field calculations. A crystal of 1-(4-methylpentanoyl)-3-propylurea (2), which shows no solid-to-solid phase transitions, was also analysed by the same method for comparison.

Journal Article↗
PubMed2003

Universal scaling behavior at the upper critical dimension of nonequilibrium continuous phase transitions.

In this work we analyze the universal scaling functions and the critical exponents at the upper critical dimension of a continuous phase transition. The consideration of the universal scaling behavior yields a decisive check of the value of the upper critical dimension. We apply our method to a nonequilibrium continuous phase transition. By focusing on the equation of state of the phase transition it is easy to extend our analysis to all equilibrium and nonequilibrium phase transitions observed numerically or experimentally.

Journal Article↗
PubMed2005

Viscosity peaks at the cholesteric-isotropic phase transitions.

In this work, the effective viscosity of the cholesteryl myristate and cholesteryl nonanoate liquid crystals is studied as a function of temperature at the region of their cholesteric-to-isotropic phase transition, where blue phases are found. Using a change of scale it is shown that the viscosity peaks that characterize these phase transitions are shape invariant, which suggests that large-scale fluctuations on the two-point correlation function give an important contribution to the observed viscosity. The consequences of this fact are investigated and, from the experimental data, the critical exponents associated with the diverging two-point correlation function are calculated. The results found for both compounds are essentially the same, being also in good agreement with the known values of the corresponding critical exponents of nematic-isotropic phase transition.

Journal Article↗
PubMed2003

Phase transition in multiprocessor scheduling.

An "easy-hard" phase transition is shown to characterize the multiprocessor scheduling problem in which one has to distribute the workload on a parallel computer such as to minimize the overall run time. The transition can be analyzed in detail by mapping it on a mean-field antiferromagnetic Potts model. The static phase transition, characterized by a vanishing ground state entropy, corresponds to a transition in the performance of practical scheduling algorithms.

Journal Article↗
PubMed2006

The phase transition behavior of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) model membrane influenced by 2,4-dichlorophenol--an FT-Raman spectroscopy study.

The effect of 2,4-dichlorophenol (DCP) on the structures and phase transitions of fully hydrated 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) liposomes was studied using FT-Raman spectroscopy. Whereas the Raman frequency shifts of the most frequently investigated bands of C-C and C-H stretching regions only indicate the main phase transition (P(beta')-L(alpha)) of the pure DPPC/water system, the Raman shift of C-H scissoring vibration at 1440 cm(-1) was found to be able to reveal the pretransition (L(beta')-P(beta')) as well. Analyzing the spectral parameters of the trans band at 1128 cm(-1), which does not overlap with DCP vibrational modes, a continuous decrease of trans conformations was found with increasing DCP concentration at 26 degrees C accompanying the phase transitions L(beta')-P(beta') and P(beta')-L(alpha). The intensity ratio of the symmetrical and asymmetrical methylene stretching bands (at 2850 cm(-1) and 2880 cm(-1)), defined as the disorder parameter by Levin [Levin, I.W., 1985. Two types of hydrocarbon chain interdigitation in sphingomielin bilayers. Biochemistry 24, 6282-6286], indicated that in the interdigitated phase (L(I)) the order is markedly high and comparable with that of L(beta). Both the phase transition P(beta')-L(alpha) in the DCP/DPPC molar ratio range of 10/100-50/100 and the phase transition L(I)-L(alpha) led to a significant increase of disordered chains and the presence of DCP molecules induced a more disordered chain region than that observed in the L(alpha) phase of DPPC. Nevertheless, it was found that the L(alpha) phase with DCP contains approximately the same amount of trans conformers than that without DCP.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed2001

Lyotropic phase transitions in phospholipids as evidenced by small-angle synchrotron X-ray scattering.

Hydration is an important factor in regulating the phase behaviour of lipids and besides affects their interactions with other compounds relevant for biological membranes. We present a reliable and fast method to detect and characterise hydration-induced phase transitions in phospholipids by means of small-angle synchrotron X-ray scattering. Films consisting of aggregations of representatives of the two important lipid classes lecithins (DPPC a, POPC and OPPC,a for abbreviations, see below) and cephalins (DPPE and DOPE) were investigated at room temperature in dependence on relative humidity. Qualitative changes in the sets of the diffraction patterns obtained in dynamic hydration/dehydration scans were taken as markers indicating the existence of lyotropic phase transitions. The efficiency of this methodology is demonstrated by illustrating the course of hydration-driven phase transitions between lamellar as well as nonlamellar phases. In detail, this was realised for chain melting in the mixed-chain lipids, POPC and OPPC, and for a novel nonlamellar-phase transition for DOPE between a disordered inverted ribbon phase designated as Palpha and the canonical H(II), phase, respectively.

Macromolecular Substances↗
PubMed2002

Chain-melting phase transition and short-range molecular interactions in phospholipid foam bilayers.

Occurrence of two-dimensional chain melting phase transition in foam bilayers was established for the first time. Microscopic horizontal foam bilayers [Newton black films (NBF)] were investigated by the microinterferometric method of Scheludko-Exerowa. The foam bilayers were formed from water-ethanol solutions of dimyristoylphosphatidylcholine (DMPC) and dipalmitoylphosphatidylcholine (DPPC) and egg phosphatidylcholine (Egg PC) and samples of amniotic fluid (AF) at different temperatures. The influence of temperature on the foam bilayer thickness h(w) and on the critical concentration Cc for formation of foam bilayer was studied. It was shown that in the range of the main phase transition the temperature dependence of h(w) and C(c) changed specifically in the case of DMPC and DPPC foam bilayers. The thickness of the foam bilayers increased with decreasing temperature in the range of the main phase transition due to the melting of hydrocarbon tails of phospholipid molecules. These changes took place at the temperatures of the bulk chain-melting phase transitions, as determined by differential scanning calorimetry (DSC) for both aqueous, and water/ethanol DMPC, DPPC, and DPPC dispersions. An effect of the 'disperse medium' on h(w) was found for foam bilayers from DPPC. The results that foam bilayers could have different thickness at different temperatures disproved the current concept that NBF acquired constant thickness at concentrations higher than C(el,cr). The data for Cc were analysed on the basis of the hole-nucleation theory of bilayer stability of Kashchiev and Exerowa. This theory considered the amphiphile bilayer as a two-dimensional ordered system with short-range molecular interactions between the first neighbour molecules (as in a crystal). The short-range molecular interactions were presented by the parameter binding energy Q of an amphiphile molecule in the bilayer. The binding energy Q of two neighbouring phospholipids was calculated for the gel (30-60 kT) and liquid crystalline state (16-18 kT) of the bilayers from DMPC, DPPC, Egg PC, AF. Concentration/temperature phase diagram of DPPC foam bilayers that defined regions of gaseous (ruptured), gel and liquid crystalline foam bilayers were drawn. The values of Q obtained for various samples were very close and vary from 5.3 x 10(-20) to 9.4 x 10(-20) (approx. 13-22 kT) which indicated that in all cases the foam bilayers were in liquid-crystalline state. This is an important result since the parameter studied-threshold concentration (threshold dilution) is crucial for a very successful assessment of the risk for respiratory distress syndrome (RDS) in newborns and could be employed in medicine for assessment of other respiratory disturbances. It is to be expected that foam bilayers from phospholipids could be used as a model for investigation of short-range forces in biological structures, of interaction between membranes, etc.

Amniotic Fluid↗
PubMed2005

Liquid-liquid phase transitions in supercooled water studied by computer simulations of various water models.

Liquid-liquid and liquid-vapor coexistence regions of various water models were determined by Monte Carlo (MC) simulations of isotherms of density fluctuation-restricted systems and by Gibbs ensemble MC simulations. All studied water models show multiple liquid-liquid phase transitions in the supercooled region: we observe two transitions of the TIP4P, TIP5P, and SPCE models and three transitions of the ST2 model. The location of these phase transitions with respect to the liquid-vapor coexistence curve and the glass temperature is highly sensitive to the water model and its implementation. We suggest that the apparent thermodynamic singularity of real liquid water in the supercooled region at about 228 K is caused by an approach to the spinodal of the first (lowest density) liquid-liquid phase transition. The well-known density maximum of liquid water at 277 K is related to the second liquid-liquid phase transition, which is located at positive pressures with a critical point close to the maximum. A possible order parameter and the universality class of liquid-liquid phase transitions in one-component fluids are discussed.

Journal Article↗
PubMed2000

Quantum phase transitions in the shastry-sutherland model for SrCu2(BO3)(2)

We investigate quantum phase transitions in the frustrated antiferromagnetic Heisenberg model for SrCu2(BO3)(2) by using the series expansion method. It is found that a novel spin-gap phase, adiabatically connected to the plaquette-singlet phase, exists between the dimer and the magnetically ordered phases known thus far. When the ratio of the competing exchange couplings alpha( = J'/J) is varied, this spin-gap phase exhibits a first- (second-) order quantum phase transition to the dimer (the magnetically ordered) phase at the critical point alpha(c1) = 0.677(2) [ alpha(c2) = 0. 86(1)]. Our results shed light on some controversial arguments about the nature of quantum phase transitions in this model.

Journal Article↗
PubMed2005

Topology and phase transitions: from an exactly solvable model to a relation between topology and thermodynamics.

The elsewhere surmized topological origin of phase transitions is given here important evidence through the analytic study of an exactly solvable model for which both topology of submanifolds of configuration space and thermodynamics are worked out. The model is a mean-field one with a k-body interaction. It undergoes a second-order phase transition for k=2 and a first-order one for k >2 . This opens a perspective for the understanding of the deep origin of first and second-order phase transitions, respectively. In particular, a remarkable theoretical result consists of a mathematical characterization of first-order transitions. Moreover, we show that a "reduced" configuration space can be defined in terms of collective variables, such that the correspondence between phase transitions and topology changes becomes one-to-one, for this model. Finally, an unusual relationship is worked out between the microscopic description of a classical N -body system and its macroscopic thermodynamic behavior. This consists of a functional dependence of thermodynamic entropy upon the Morse indexes of the critical points (saddles) of the constant energy hypersurfaces of the microscopic 2N-dimensional phase space. Thus phase space (and configuration space) topology is directly related to thermodynamics.

Journal Article↗
PubMed2005

Thermal-induced phase transitions in self-assembled mesostructured films studied by small-angle X-ray scattering.

Two examples of phase transition in self-assembled mesostructured hybrid thin films are reported. The materials have been synthesized using tetraethoxysilane as the silica source hydrolyzed with or without the addition of methyltriethoxysilane. The combined use of transmission electron microscopy, small-angle X-ray scattering and computer simulation has been introduced to achieve a clear identification of the organized phases. A structural study of the self-assembled mesophases as a function of thermal treatment has allowed the overall phase transition to be followed. The initial symmetries of mesophases in as-deposited films have been linked to those observed in samples after thermal treatment. The monodimensional shrinkage of silica films during calcination has induced a phase transition from face-centered orthorhombic to body-centered cubic. In hybrid films, instead, the phase transition has not involved a change in the unit cell but a contraction of the cell parameter normal to the substrate.

Journal Article↗